X-ray beam collimators form an unshaped X-ray beam into a specific, desired shape. Fan beam collimators simply use a lead sheet or other blocking material with a slit in it to form a fan beam. See U.S. Pat. No. 3,808,444. Chopper wheel collimators use a plurality of generally radial slots to chop a fan beam into pencil beams. See the above patent and U.S. Pat. No. 5,181,234. These are often called flying spot scanners because the pencil beam moves cross-wise back and forth or up and down as the chopper wheel rotates. See also U.S. Pat. No. 4,745,631. A rotating wheel collimator has an X-ray source at its center and a plurality of radial channels extending outward to create pencil beams. See U.S. Pat. No. 5,493,546. Two other designs use helical slits to create a continuous or flying spot pencil beam from a fan beam. In one approach, U.S. Pat. No. 4,745,631, there are helical slits on the surface of a cylinder which create a fly spot pencil beam as the cylinder rotates. In another approach there is a helical slit on the surface of the cylinder. See U.S. Pat. Nos. 5,493,596 and 5,528,656.